Wireless communication method for multi-user transmission scheduling, and wireless communication terminal using same

ABSTRACT

The present invention relates to a wireless communication terminal and a wireless communication method for efficiently scheduling uplink multi-user transmission. 
     To this end, provided are a base wireless communication terminal, including: a transceiver configured to transmit and receive a wireless signal; and a processor configured to control an operation of the transceiver, wherein the processor selects an access category for transmitting a trigger frame which solicits an uplink multi-user transmission, performs a backoff procedure for transmitting the trigger frame based on the selected access category, and transmits the trigger frame when a backoff counter of the backoff procedure expires and a wireless communication method using the same.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/736,968 filed on May 9, 2018, which is the U.S. National Stage ofInternational Patent Application No. PCT/KR2016/006415 filed on Jun. 16,2016, which claims the priority to Korean Patent Application No.10-2015-0085451 filed in the Korean Intellectual Property Office on Jun.16, 2015, Korean Patent Application No. 10-2015-0092534 filed in theKorean Intellectual Property Office on Jun. 29, 2015 and Korean PatentApplication No. 10-2016-0059090 filed in the Korean IntellectualProperty Office on May 13, 2016, the entire contents of which areincorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a wireless communication method formulti-user transmission scheduling and a wireless communication terminalusing the same, and more particularly, to a wireless communicationmethod and a wireless communication terminal for efficiently schedulingsimultaneous transmission of a plurality of terminals.

BACKGROUND ART

In recent years, with supply expansion of mobile apparatuses, a wirelessLAN technology that can provide a rapid wireless Internet service to themobile apparatuses has been significantly spotlighted. The wireless LANtechnology allows mobile apparatuses including a smart phone, a smartpad, a laptop computer, a portable multimedia player, an embeddedapparatus, and the like to wirelessly access the Internet in home or acompany or a specific service providing area based on a wirelesscommunication technology in a short range.

Institute of Electrical and Electronics Engineers (IEEE) 802.11 hascommercialized or developed various technological standards since aninitial wireless LAN technology is supported using frequencies of 2.4GHz. First, the IEEE 802.11b supports a communication speed of a maximumof 11 Mbps while using frequencies of a 2.4 GHz band. IEEE 802.11a whichis commercialized after the IEEE 802.11b uses frequencies of not the 2.4GHz band but a 5 GHz band to reduce an influence by interference ascompared with the frequencies of the 2.4 GHz band which aresignificantly congested and improves the communication speed up to amaximum of 54 Mbps by using an OFDM technology. However, the IEEE802.11a has a disadvantage in that a communication distance is shorterthan the IEEE 802.11b. In addition, IEEE 802.11g uses the frequencies ofthe 2.4 GHz band similarly to the IEEE 802.11b to implement thecommunication speed of a maximum of 54 Mbps and satisfies backwardcompatibility to significantly come into the spotlight and further, issuperior to the IEEE 802.11a in terms of the communication distance.

Moreover, as a technology standard established to overcome a limitationof the communication speed which is pointed out as a weak point in awireless LAN, IEEE 802.11n has been provided. The IEEE 802.11n aims atincreasing the speed and reliability of a network and extending anoperating distance of a wireless network. In more detail, the IEEE802.11n supports a high throughput (HT) in which a data processing speedis a maximum of 540 Mbps or more and further, is based on a multipleinputs and multiple outputs (MIMO) technology in which multiple antennasare used at both sides of a transmitting unit and a receiving unit inorder to minimize a transmission error and optimize a data speed.Further, the standard can use a coding scheme that transmits multiplecopies which overlap with each other in order to increase datareliability.

As the supply of the wireless LAN is activated and further, applicationsusing the wireless LAN are diversified, the need for new wireless LANsystems for supporting a higher throughput (very high throughput (VHT))than the data processing speed supported by the IEEE 802.11n has comeinto the spotlight. Among them, IEEE 802.11ac supports a wide bandwidth(80 to 160 MHz) in the 5 GHz frequencies. The IEEE 802.11ac standard isdefined only in the 5 GHz band, but initial 11ac chipsets will supporteven operations in the 2.4 GHz band for the backward compatibility withthe existing 2.4 GHz band products. Theoretically, according to thestandard, wireless LAN speeds of multiple stations are enabled up to aminimum of 1 Gbps and a maximum single link speed is enabled up to aminimum of 500 Mbps. This is achieved by extending concepts of awireless interface accepted by 802.11n, such as a wider wirelessfrequency bandwidth (a maximum of 160 MHz), more MIMO spatial streams (amaximum of 8), multi-user MIMO, and high-density modulation (a maximumof 256 QAM). Further, as a scheme that transmits data by using a 60 GHzband instead of the existing 2.4 GHz/5 GHz, IEEE 802.11ad has beenprovided. The IEEE 802.11ad is a transmission standard that provides aspeed of a maximum of 7 Gbps by using a beamforming technology and issuitable for high bit rate moving picture streaming such as massive dataor non-compression HD video. However, since it is difficult for the 60GHz frequency band to pass through an obstacle, it is disadvantageous inthat the 60 GHz frequency band can be used only among devices in ashort-distance space.

Meanwhile, in recent years, as next-generation wireless LAN standardsafter the 802.11ac and 802.11ad, discussion for providing ahigh-efficiency and high-performance wireless LAN communicationtechnology in a high-density environment is continuously performed. Thatis, in a next-generation wireless LAN environment, communication havinghigh frequency efficiency needs to be provided indoors/outdoors underthe presence of high-density stations and access points (APs) andvarious technologies for implementing the communication are required.

DISCLOSURE Technical Problem

The present invention has an object to providehigh-efficiency/high-performance wireless LAN communication in ahigh-density environment as described above.

The present invention has an object to perform efficient scheduling ofan uplink/downlink multi-user transmission.

In addition, the present invention has an object to provide an efficientscheduling method for channel access of each terminal in a situationwhere a multi-user transmission and a single-user transmission aremixed.

Technical Solution

In order to achieve the objects, the present invention provides awireless communication method and a wireless communication terminal asbelow.

First, an exemplary embodiment of the present invention provides a basewireless communication terminal comprising a processor and atransceiver, wherein the processor selects an access category fortransmitting a trigger frame which solicits an uplink multi-usertransmission, performs a backoff procedure for transmitting the triggerframe based on the selected access category, and transmits the triggerframe when a backoff counter of the backoff procedure expires.

The processor may determines a size of a contention window for thebackoff procedure using parameters of the selected access category,obtain a backoff counter within the determined contention window, andperform the backoff procedure using the obtained backoff counter.

The parameter may include a minimum contention window value and amaximum contention window value, and a size of a contention window forthe backoff procedure may be determined between a minimum contentionwindow value and a maximum contention window value of the selectedaccess category.

The access category for transmitting the trigger frame may have a higherpriority than access categories of data to be transmitted to anotherterminal.

The processor may determine that the uplink multi-user transmission issuccessful and transmits a block ACK when uplink data is received fromat least one terminal indicated by the trigger frame.

The processor may determine that the uplink multi-user transmission hasfailed and retransmit the trigger frame when no uplink datacorresponding to the trigger frame has received.

The processor may increase a size of a contention window based on theaccess category for transmitting the trigger frame, obtain a new backoffcounter within an increased contention window, and perform a backoffprocedure for retransmitting the trigger frame using the new backoffcounter.

The base wireless communication terminal may receive a buffer statusreport of at least one terminal through the transceiver, and perform abackoff procedure for transmitting the trigger frame when receivedbuffer status report information is a predetermined amount or more.

The processor may generate a virtual queue for transmitting the triggerframe using the received buffer status report, and determine whether totransmit the trigger frame based on internal contention between anaccess category queue for a downlink single-user transmission of thebase wireless communication terminal and the virtual queue.

The processor may assign backoff counters respectively corresponding tothe access category queue and the virtual queue, the backoff countersbeing respectively assigned based on parameters of an access categoryset in a corresponding queue, and transmit the trigger frame when abackoff counter corresponding to the virtual queue expires.

In addition, an exemplary embodiment of the present invention provides awireless communication method of a base wireless communication terminal,the method including: selecting an access category for transmitting atrigger frame which solicits an uplink multi-user transmission;performing a backoff procedure for transmitting the trigger frame basedon the selected access category; and transmitting the trigger frame whena backoff counter of the backoff procedure expires.

Advantageous Effects

According to an embodiment of the present invention, efficient uplinkmulti-user transmission scheduling is possible in a contention-basedchannel access system.

According to an embodiment of the present invention, it is possible toincrease the total resource utilization rate in the contention-basedchannel access system and improve the performance of the wireless LANsystem.

DESCRIPTION OF DRAWINGS

FIG. 1 illustrates a wireless LAN system according to an embodiment ofthe present invention.

FIG. 2 illustrates a wireless LAN system according to another embodimentof the present invention.

FIG. 3 illustrates a configuration of a station according to anembodiment of the present invention.

FIG. 4 illustrates a configuration of an access point according to anembodiment of the present invention.

FIG. 5 schematically illustrates a process in which a STA and an AP seta link.

FIG. 6 illustrates a carrier sense multiple access (CSMA)/collisionavoidance (CA) method used in wireless LAN communication.

FIG. 7 illustrates a method for performing a distributed coordinationfunction (DCF) using a request to send (RTS) frame and a clear to send(CTS) frame.

FIGS. 8 and 9 illustrate an embodiment of an uplink multi-usertransmission process of a non-legacy wireless LAN system.

FIGS. 10 to 12 illustrate various embodiments of channel access in arandom access based uplink multi-user transmission process.

FIGS. 13 to 15 illustrate an embodiment in which the embodiments ofFIGS. 10 to 12 are extended to a general uplink multi-user transmissionprocess.

FIGS. 16 and 17 illustrate embodiments of channel access in an uplinkmulti-user transmission process using a wideband channel.

FIG. 18 illustrates an embodiment of an enhanced distributed channelaccess (EDCA).

FIG. 19 illustrates an embodiment of a downlink multi-user transmissionprocess.

FIGS. 20 to 22 illustrate a channel access method when a transmission ofsome data has failed in the downlink multi-user transmission process.

FIGS. 23 and 24 illustrate a channel access method when a datatransmission of primary access category has failed in the downlinkmulti-user transmission process.

FIGS. 25 and 26 illustrate an embodiment of an EDCA including multi-usertransmission.

DETAILED DESCRIPTION OF THE INVENTION

Terms used in the specification adopt general terms which are currentlywidely used by considering functions in the present invention, but theterms may be changed depending on an intention of those skilled in theart, customs, and emergence of new technology. Further, in a specificcase, there is a term arbitrarily selected by an applicant and in thiscase, a meaning thereof will be described in a corresponding descriptionpart of the invention. Accordingly, it should be revealed that a termused in the specification should be analyzed based on not just a name ofthe term but a substantial meaning of the term and contents throughoutthe specification.

Throughout this specification and the claims that follow, when it isdescribed that an element is “coupled” to another element, the elementmay be “directly coupled” to the other element or “electrically coupled”to the other element through a third element. Further, unless explicitlydescribed to the contrary, the word “comprise” and variations such as“comprises” or “comprising”, will be understood to imply the inclusionof stated elements but not the exclusion of any other elements.Moreover, limitations such as “or more” or “or less” based on a specificthreshold may be appropriately substituted with “more than” or “lessthan”, respectively.

This application claims priority to and the benefit of Korean PatentApplication Nos. 10-2015-0085451, 10-2015-0092534 and 10-2016-0059090filed in the Korean Intellectual Property Office and the embodiments andmentioned items described in the respective application, which forms thebasis of the priority, shall be included in the Detailed Description ofthe present application.

FIG. 1 is a diagram illustrating a wireless LAN system according to anembodiment of the present invention. The wireless LAN system includesone or more basic service sets (BSS) and the BSS represents a set ofapparatuses which are successfully synchronized with each other tocommunicate with each other. In general, the BSS may be classified intoan infrastructure BSS and an independent BSS (IBSS) and FIG. 1illustrates the infrastructure BSS between them.

As illustrated in FIG. 1, the infrastructure BSS (BSS1 and BSS2)includes one or more stations STA1, STA2, STA3, STA4, and STA5, accesspoints PCP/AP-1 and PCP/AP-2 which are stations providing a distributionservice, and a distribution system (DS) connecting the multiple accesspoints PCP/AP-1 and PCP/AP-2.

The station (STA) is a predetermined device including medium accesscontrol (MAC) following a regulation of an IEEE 802.11 standard and aphysical layer interface for a wireless medium, and includes both anon-access point (non-AP) station and an access point (AP) in a broadsense. Further, in the present specification, a term ‘terminal’ may beused to refer to a non-AP STA, or an AP, or to both terms. A station forwireless communication includes a processor and a transceiver andaccording to the embodiment, may further include a user interface unitand a display unit. The processor may generate a frame to be transmittedthrough a wireless network or process a frame received through thewireless network and besides, perform various processing for controllingthe station. In addition, the transceiver is functionally connected withthe processor and transmits and receives frames through the wirelessnetwork for the station.

The access point (AP) is an entity that provides access to thedistribution system (DS) via wireless medium for the station associatedtherewith. In the infrastructure BSS, communication among non-APstations is, in principle, performed via the AP, but when a direct linkis configured, direct communication is enabled even among the non-APstations. Meanwhile, in the present invention, the AP is used as aconcept including a personal BSS coordination point (PCP) and mayinclude concepts including a centralized controller, a base station(BS), a node-B, a base transceiver system (BTS), and a site controllerin a broad sense. In the present invention, an AP may also be referredto as a base wireless communication terminal. The base wirelesscommunication terminal may be used as a term which includes an AP, abase station, an eNB (i.e. eNodeB) and a transmission point (TP) in abroad sense. In addition, the base wireless communication terminal mayinclude various types of wireless communication terminals that allocatemedium resources and perform scheduling in communication with aplurality of wireless communication terminals.

A plurality of infrastructure BSSs may be connected with each otherthrough the distribution system (DS). In this case, a plurality of BSSsconnected through the distribution system is referred to as an extendedservice set (ESS).

FIG. 2 illustrates an independent BSS which is a wireless LAN systemaccording to another embodiment of the present invention. In theembodiment of FIG. 2, duplicative description of parts, which are thesame as or correspond to the embodiment of FIG. 1, will be omitted.

Since a BSS3 illustrated in FIG. 2 is the independent BSS and does notinclude the AP, all stations STA6 and STA7 are not connected with theAP. The independent BSS is not permitted to access the distributionsystem and forms a self-contained network. In the independent BSS, therespective stations STA6 and STA7 may be directly connected with eachother.

FIG. 3 is a block diagram illustrating a configuration of a station 100according to an embodiment of the present invention.

As illustrated in FIG. 3, the station 100 according to the embodiment ofthe present invention may include a processor 110, a transceiver 120, auser interface unit 140, a display unit 150, and a memory 160.

First, the transceiver 120 transmits and receives a wireless signal suchas a wireless LAN packet, or the like and may be embedded in the station100 or provided as an exterior. According to the embodiment, thetransceiver 120 may include at least one transmit/receive module usingdifferent frequency bands. For example, the transceiver 120 may includetransmit/receive modules having different frequency bands such as 2.4GHz, 5 GHz, and 60 GHz. According to an embodiment, the station 100 mayinclude a transmit/receive module using a frequency band of 6 GHz ormore and a transmit/receive module using a frequency band of 6 GHz orless. The respective transmit/receive modules may perform wirelesscommunication with the AP or an external station according to a wirelessLAN standard of a frequency band supported by the correspondingtransmit/receive module. The transceiver 120 may operate only onetransmit/receive module at a time or simultaneously operate multipletransmit/receive modules together according to the performance andrequirements of the station 100. When the station 100 includes aplurality of transmit/receive modules, each transmit/receive module maybe implemented by independent elements or a plurality of modules may beintegrated into one chip. In an embodiment of the present invention, thetransceiver 120 may represent a radio frequency (RF) transceiver modulefor processing an RF signal.

Next, the user interface unit 140 includes various types of input/outputmeans provided in the station 100. That is, the user interface unit 140may receive a user input by using various input means and the processor110 may control the station 100 based on the received user input.Further, the user interface unit 140 may perform output based on acommand of the processor 110 by using various output means.

Next, the display unit 150 outputs an image on a display screen. Thedisplay unit 150 may output various display objects such as contentsexecuted by the processor 110 or a user interface based on a controlcommand of the processor 110, and the like. Further, the memory 160stores a control program used in the station 100 and various resultingdata. The control program may include an access program required for thestation 100 to access the AP or the external station.

The processor 110 of the present invention may execute various commandsor programs and process data in the station 100. Further, the processor110 may control the respective units of the station 100 and control datatransmission/reception among the units. According to the embodiment ofthe present invention, the processor 110 may execute the program foraccessing the AP stored in the memory 160 and receive a communicationconfiguration message transmitted by the AP. Further, the processor 110may read information on a priority condition of the station 100 includedin the communication configuration message and request the access to theAP based on the information on the priority condition of the station100. The processor 110 of the present invention may represent a maincontrol unit of the station 100 and according to the embodiment, theprocessor 110 may represent a control unit for individually controllingsome component of the station 100, for example, the transceiver 120, andthe like. That is, the processor 110 may be a modem or amodulator/demodulator for modulating and demodulating wireless signalstransmitted to and received from the transceiver 120. The processor 110controls various operations of wireless signal transmission/reception ofthe station 100 according to the embodiment of the present invention. Adetailed embodiment thereof will be described below.

The station 100 illustrated in FIG. 3 is a block diagram according to anembodiment of the present invention, where separate blocks areillustrated as logically distinguished elements of the device.Accordingly, the elements of the device may be mounted in a single chipor multiple chips depending on design of the device. For example, theprocessor 110 and the transceiver 120 may be implemented while beingintegrated into a single chip or implemented as a separate chip.Further, in the embodiment of the present invention, some components ofthe station 100, for example, the user interface unit 140 and thedisplay unit 150 may be optionally provided in the station 100.

FIG. 4 is a block diagram illustrating a configuration of an AP 200according to an embodiment of the present invention.

As illustrated in FIG. 4, the AP 200 according to the embodiment of thepresent invention may include a processor 210, a transceiver 220, and amemory 260. In FIG. 4, among the components of the AP 200, duplicativedescription of parts which are the same as or correspond to thecomponents of the station 100 of FIG. 2 will be omitted.

Referring to FIG. 4, the AP 200 according to the present inventionincludes the transceiver 220 for operating the BSS in at least onefrequency band. As described in the embodiment of FIG. 3, thetransceiver 220 of the AP 200 may also include a plurality oftransmit/receive modules using different frequency bands. That is, theAP 200 according to the embodiment of the present invention may includetwo or more transmit/receive modules among different frequency bands,for example, 2.4 GHz, 5 GHz, and 60 GHz together. Preferably, the AP 200may include a transmit/receive module using a frequency band of 6 GHz ormore and a transmit/receive module using a frequency band of 6 GHz orless. The respective transmit/receive modules may perform wirelesscommunication with the station according to a wireless LAN standard of afrequency band supported by the corresponding transmit/receive module.The transceiver 220 may operate only one transmit/receive module at atime or simultaneously operate multiple transmit/receive modulestogether according to the performance and requirements of the AP 200. Inan embodiment of the present invention, the transceiver 220 mayrepresent a radio frequency (RF) transceiver module for processing an RFsignal.

Next, the memory 260 stores a control program used in the AP 200 andvarious resulting data. The control program may include an accessprogram for managing the access of the station. Further, the processor210 may control the respective units of the AP 200 and control datatransmission/reception among the units. According to the embodiment ofthe present invention, the processor 210 may execute the program foraccessing the station stored in the memory 260 and transmitcommunication configuration messages for one or more stations. In thiscase, the communication configuration messages may include informationabout access priority conditions of the respective stations. Further,the processor 210 performs an access configuration according to anaccess request of the station. According to an embodiment, the processor210 may be a modem or a modulator/demodulator for modulating anddemodulating wireless signals transmitted to and received from thetransceiver 220. The processor 210 controls various operations such aswireless signal transmission/reception of the AP 200 according to theembodiment of the present invention. A detailed embodiment thereof willbe described below.

FIG. 5 is a diagram schematically illustrating a process in which a STAsets a link with an AP.

Referring to FIG. 5, the link between the STA 100 and the AP 200 is setthrough three steps of scanning, authentication, and association in abroad way. First, the scanning step is a step in which the STA 100obtains access information of BSS operated by the AP 200. A method forperforming the scanning includes a passive scanning method in which theAP 200 obtains information by using a beacon message (S101) which isperiodically transmitted and an active scanning method in which the STA100 transmits a probe request to the AP (S103) and obtains accessinformation by receiving a probe response from the AP (S105).

The STA 100 that successfully receives wireless access information inthe scanning step performs the authentication step by transmitting anauthentication request (S107 a) and receiving an authentication responsefrom the AP 200 (S107 b). After the authentication step is performed,the STA 100 performs the association step by transmitting an associationrequest (S109 a) and receiving an association response from the AP 200(S109 b). In this specification, an association basically means awireless association, but the present invention is not limited thereto,and the association may include both the wireless association and awired association in a broad sense.

Meanwhile, an 802.1X based authentication step (S111) and an IP addressobtaining step (S113) through DHCP may be additionally performed. InFIG. 5, the authentication server 300 is a server that processes 802.1Xbased authentication with the STA 100 and may be present in physicalassociation with the AP 200 or present as a separate server.

FIG. 6 is a diagram illustrating a carrier sense multiple access(CSMA)/collision avoidance (CA) method used in wireless LANcommunication.

A terminal that performs a wireless LAN communication checks whether achannel is busy by performing carrier sensing before transmitting data.When a wireless signal having a predetermined strength or more issensed, it is determined that the corresponding channel is busy and theterminal delays the access to the corresponding channel. Such a processis referred to as clear channel assessment (CCA) and a level to decidewhether the corresponding signal is sensed is referred to as a CCAthreshold. When a wireless signal having the CCA threshold or more,which is received by the terminal, indicates the corresponding terminalas a receiver, the terminal processes the received wireless signal.Meanwhile, when a wireless signal is not sensed in the correspondingchannel or a wireless signal having a strength smaller than the CCAthreshold is sensed, it is determined that the channel is idle.

When it is determined that the channel is idle, each terminal havingdata to be transmitted performs a backoff procedure after an interframespace (IFS) time depending on a situation of each terminal, forinstance, an arbitration IFS (AIFS), a PCF IFS (PIFS), or the likeelapses. According to the embodiment, the AIFS may be used as acomponent which substitutes for the existing DCF IFS (DIFS). Eachterminal stands by while decreasing slot time(s) as long as a randomnumber assigned to the corresponding terminal during an interval of anidle state of the channel and a terminal that completely exhausts theslot time(s) attempts to access the corresponding channel. As such, aninterval in which each terminal performs the backoff procedure isreferred to as a contention window interval.

When a specific terminal successfully accesses the channel, thecorresponding terminal may transmit data through the channel. However,when the terminal which attempts the access collides with anotherterminal, the terminals which collide with each other are assigned withnew random numbers, respectively to perform the backoff procedure again.According to an embodiment, a random number newly assigned to eachterminal may be decided within a range (2*CW) which is twice larger thana range (a contention window, CW) of a random number which thecorresponding terminal is previously assigned. Meanwhile, each terminalattempts the access by performing the backoff procedure again in a nextcontention window interval and in this case, each terminal performs thebackoff procedure from slot time(s) which remained in the previouscontention window interval. By such a method, the respective terminalsthat perform the wireless LAN communication may avoid a mutual collisionfor a specific channel.

FIG. 7 is a diagram illustrating a method for performing a distributedcoordination function using a request to send (RTS) frame and a clear tosend (CTS) frame.

The AP and STAs in the BSS contend in order to obtain an authority fortransmitting data. When data transmission at the previous step iscompleted, each terminal having data to be transmitted performs abackoff procedure while decreasing a backoff counter (alternatively, abackoff timer) of a random number assigned to each terminal after anAFIS time. A transmitting terminal in which the backoff counter expirestransmits the request to send (RTS) frame to notify that correspondingterminal has data to transmit. According to an exemplary embodiment ofFIG. 7, STA1 which holds a lead in contention with minimum backoff maytransmit the RTS frame after the backoff counter expires. The RTS frameincludes information on a receiver address, a transmitter address, andduration. A receiving terminal (i.e., the AP in FIG. 7) that receivesthe RTS frame transmits the clear to send (CTS) frame after waiting fora short IFS (SIFS) time to notify that the data transmission isavailable to the transmitting terminal STA1. The CTS frame includes theinformation on a receiver address and duration. In this case, thereceiver address of the CTS frame may be set identically to atransmitter address of the RTS frame corresponding thereto, that is, anaddress of the transmitting terminal STA1.

The transmitting terminal STA1 that receives the CTS frame transmits thedata after a SIFS time. When the data transmission is completed, thereceiving terminal AP transmits an acknowledgment (ACK) frame after aSIFS time to notify that the data transmission is completed. When thetransmitting terminal receives the ACK frame within a predeterminedtime, the transmitting terminal regards that the data transmission issuccessful. However, when the transmitting terminal does not receive theACK frame within the predetermined time, the transmitting terminalregards that the data transmission is failed. Meanwhile, adjacentterminals that receive at least one of the RTS frame and the CTS framein the course of the transmission procedure set a network allocationvector (NAV) and do not perform data transmission until the set NAV isterminated. In this case, the NAV of each terminal may be set based on aduration field of the received RTS frame or CTS frame.

In the course of the aforementioned data transmission procedure, whenthe RTS frame or CTS frame of the terminals is not normally transferredto a target terminal (i.e., a terminal of the receiver address) due to asituation such as interference or a collision, a subsequent process issuspended. The transmitting terminal STA1 that transmitted the RTS frameregards that the data transmission is unavailable and participates in anext contention by being assigned with a new random number. In thiscase, the newly assigned random number may be determined within a range(2*CW) twice larger than a previous predetermined random number range (acontention window, CW).

Uplink Multi-User Transmission

When using orthogonal frequency division multiple access (OFDMA) ormulti-input multi-output (MIMO), one wireless communication terminal cansimultaneously transmit data to a plurality of wireless communicationterminals. Further, one wireless communication terminal cansimultaneously receive data from a plurality of wireless communicationterminals. For example, a downlink multi-user (DL-MU) transmission inwhich an AP simultaneously transmits data to a plurality of STAs, and anuplink multi-user (UL-MU) transmission in which a plurality of STAssimultaneously transmit data to the AP may be performed.

In order to perform the UL-MU transmission, the channel to be used andthe transmission start time of each STA that performs uplinktransmission should be adjusted. In order to efficiently schedule theUL-MU transmission, state information of each STA needs to betransmitted to the AP. According to an embodiment of the presentinvention, information for scheduling of a UL-MU transmission may beindicated through a predetermined field of a preamble of a packet and/ora predetermined field of a MAC header. For example, a STA may indicateinformation for UL-MU transmission scheduling through a predeterminedfield of a preamble or a MAC header of an uplink transmission packet,and may transmit the information to an AP. In this case, the informationfor UL-MU transmission scheduling includes at least one of buffer statusinformation of each STA, channel state information measured by each STA.The buffer status information of the STA may indicate at least one ofwhether the STA has uplink data to be transmitted, the access category(AC) of the uplink data and the size (or the transmission time) of theuplink data.

According to an embodiment of the present invention, the UL-MUtransmission process may be managed by the AP. The UL-MU transmissionmay be performed in response to a trigger frame transmitted by the AP.The STAs simultaneously transmit uplink data a predetermined IFS timeafter receiving the trigger frame. The trigger frame indicates the datatransmission time point of the uplink STAs and may inform the channel(or subchannel) information allocated to the uplink STAs. When the APtransmits the trigger frame, a plurality of STAs transmit uplink datathrough the respective allocated subcarriers at a time point designatedby the trigger frame. After the uplink data transmission is completed,the AP transmits an ACK to the STAs that have successfully transmittedthe uplink data. In this case, the AP may transmit a predeterminedmulti-STA block ACK (M-BA) as an ACK for a plurality of STAs.

In the non-legacy wireless LAN system, a specific number, for example,26, 52, or 106 tones may be used as a resource unit (RU) for asubchannel-based access in a channel of 20 MHz band. Accordingly, thetrigger frame may indicate identification information of each STAparticipating in the UL-MU transmission and information of the allocatedresource unit. The identification information of the STA includes atleast one of an association ID (AID), a partial AID, and a MAC addressof the STA. Further, the information of the resource unit includes thesize and placement information of the resource unit.

On the other hand, in the non-legacy wireless LAN system, a UL-MUtransmission may be performed based on a contention of a plurality ofSTAs for a particular resource unit. For example, if an AID field valuefor a particular resource unit is set to a specific value (e.g., 0) thatis not assigned to STAs, a plurality of STAs may attempt random access(RA) for the corresponding resource unit.

FIGS. 8 and 9 illustrate an embodiment of an uplink multi-usertransmission process of a non-legacy wireless LAN system.

First, referring to FIG. 8, an AP transmits a trigger frame 310 forinitiating a UL-MU transmission process. The AP may perform a separatebackoff procedure for transmitting the trigger frame 310. When thebackoff procedure for transmitting the trigger frame 310 expires in thecontention window interval 42, the AP transmits the trigger frame 310.STAs receive the trigger frame 310 transmitted by the AP and transmituplink multi-user data 320, that is, the uplink multi-user PLCP protocoldata unit (UL MU PPPU) in response thereto. The uplink multi-user data320 may be transmitted in a form including at least one of OFDMA andMU-MIMO. When the transmission of the uplink multi-user data 320 issuccessful, the AP transmits an M-BA 330 in response thereto. The M-BA330 includes ACK information for STAs that have succeeded intransmitting the uplink multi-user data 320. In the embodiment of FIG.8, STA1, STA2, and STA3 succeed in uplink data transmission in responseto the trigger frame 310, and the AP transmits ACK information for STA1,STA2, and STA3 via the M-BA 330.

After the UL-MU transmission process is completed, the AP obtains a newbackoff counter for contention in the next contention window intervals44 and 46. In this case, the AP obtains a backoff counter within acontention window determined based on an access category of the nextdata to be transmitted. The AP contends with STAs based on the newbackoff counter and accesses the channel. In the embodiment of FIG. 8,STA5 has won the contention in the next contention window interval 44 ofthe UL-MU transmission process. Accordingly, the STA5 transmits uplinkdata 340 to the AP, and the AP transmits an ACK 346 in response thereto.Also, in the next contention window interval 46, the AP has won thecontention. Accordingly, the AP transmits downlink data 350 to the STA2,and the STA2 transmits an ACK 356 in response thereto.

FIG. 9 illustrates an embodiment in which a transmission of some uplinkdata has failed in the UL-MU transmission process. In the embodiment ofFIG. 9, duplicated descriptions of parts which are the same orcorresponding to those of the embodiment of FIG. 8 will be omitted.

Referring to FIG. 9, the AP transmits a TF-R 312. In an embodiment ofthe present invention, the TF-R 312 represents a random access basedtrigger frame. That is, the TF-R 312 triggers an uplink multi-user datatransmission by allocating some or all of the resources for randomaccess. The AP may set an AID field value for a specific resource unitto a predetermined value (for example, 0), to allocate the correspondingresource unit for random access. When a backoff procedure fortransmitting the TF-R 312 expires in the contention window interval 42,the AP transmits the TF-R 312. The STAs receive the TF-R 312 transmittedby the AP and transmit uplink multi-user data 322 in response thereto.

In this case, the uplink multi-user data 322 transmitted by the STAs mayinclude random access uplink data. The STAs participating in the randomaccess UL-MU transmission transmit uplink data through a resource unitallocated for random access by the TF-R 312. Since the resource unitallocated for random access is not assigned to a specific STA, aplurality of STAs may transmit uplink data at the same time and acollision may occur. In the example of FIG. 9, STA2 and STA4 transmituplink data through the same resource unit resulting in collision, andSTA3 and STA5 transmit uplink data through the same resource unitresulting in collision. However, STA1 has successfully transmitteduplink data to the AP. As described above, in the process oftransmitting the uplink multi-user data 322, a transmission of only someuplink data may be successful and a transmission of the remaining uplinkdata may have failed.

For efficient scheduling of the UL-MU transmission, various parametersto be used in a series of transmission processes should be determined.For example, the size of the contention window used in the backoffprocedure for transmitting the trigger frame should be determined. Also,as described above, a criterion for determining whether or not thetransmission of the uplink multi-user data 322 is successful should beestablished. In addition, the succeeding operation and the backoffmethod according to the success or failure determination should bedefined.

According to the embodiment of the present invention, the AP mayconsider that the transmission process is successful even when a part ofdata is successfully transmitted in the transmission of the uplinkmulti-user data 322. That is, when uplink data is received from at leastone of the STAs indicated by the trigger frame, the AP determines thatthe UL-MU transmission process is successful. Thus, the AP transmits anM-BA 330 in response to receiving the uplink multi-user data 322. In theembodiment of FIG. 9, STA1 succeeds in uplink data transmission inresponse to the TF-R 312, and the AP transmits ACK information for theSTA1 via the M-BA 330. Meanwhile, FIG. 9 illustrates an embodimentincluding a random access based UL-MU transmission. However, the presentinvention is not limited thereto, and the success/failure determinationmethod may be applied to other types of UL-MU transmission processes inthe same way.

Since the UL-MU transmission process has been determined to besuccessful, the AP does not increase the size of the contention windowto be used in the backoff procedures in the next contention windowintervals 44 and 46. That is, the AP obtains a new backoff counterwithin a contention window determined based on the access category ofthe next data to be transmitted, as in the embodiment of FIG. 8. The APcontends with the STAs based on the new backoff counter and accesses thechannel.

FIGS. 10 to 12 illustrate various embodiments of channel access in arandom access based uplink multi-user transmission process. Morespecifically, the embodiments of FIGS. 10 to 12 illustrate a schedulingmethod when the AP does not receive any uplink data corresponding to theTF-R. According to the embodiment of the present invention, when the APdoes not receive any uplink data corresponding to the trigger frame, itdetermines that the UL-MU transmission process has failed.

First, according to the embodiment of FIG. 10, the AP may performretransmission of the TF-R 312 when the UL-MU transmission process bythe TF-R 312 has failed. For retransmitting the TF-R 312, the AP obtainsa new backoff counter. In this case, the new backoff counter may bedetermined within a range of twice the contention window used inobtaining the previous backoff counter. That is, when the UL-MUtransmission process has failed, the AP doubles the size of thecontention window to be used in the backoff procedure of the nextcontention window intervals 44 and 46. In the contention windowintervals 44 and 46, the AP performs a backoff procedure to retransmitthe TF-R 312 based on the new backoff counter. The retransmission of theTF-R 312 may be performed until the retransmission is successful withina preset retransmission limit.

In the embodiment of FIG. 10, the UL-MU transmission procedure hasfailed and the next contention window interval 44 starts after anextended IFS (EIFS) time. The AP and STAs contend in the contentionwindow interval 44, and STA5 wins the contention. Accordingly, the STA5transmits uplink data 340 to the AP, and the AP transmits an ACK 346 inresponse thereto. In the next contention window interval 46, the AP winsthe contention, and the AP retransmits the TF-R 312. In response to theretransmission of the TF-R 312, the STAs transmit uplink multi-user data323. In the embodiment of FIG. 10, STA1 and STA3 succeeded in the uplinkdata transmission in response to the retransmitted TF-R 312. When uplinkdata is received from at least one STA in response to the TF-R 312, theAP determines that the UL-MU transmission process is successful.Accordingly, the AP transmits ACK information for STA1 and STA3 via theM-BA 330.

FIG. 11 illustrates an embodiment of a channel access according toanother embodiment of the present invention. In the embodiment of FIG.11, duplicated descriptions of parts which are the same or correspondingto those of the embodiment of FIG. 10 will be omitted.

According to the embodiment of FIG. 11, the AP may increase the size ofthe contention window without performing retransmission of the TF-R 312when the UL-MU transmission process by the TF-R 312 has failed. When theUL-MU transmission process has failed, the AP may attempt any one of adownlink single-user transmission, a downlink multi-user transmission,and a transmission of a new trigger frame in the next contention windowinterval. When attempting a downlink single-user transmission or adownlink multi-user transmission, the AP obtains a new backoff counterby doubling the size of the contention window based on the accesscategory of data to be transmitted. When transmitting a new triggerframe, the AP obtains a new backoff counter by doubling the size of theexisting contention window based on the access category for the triggerframe. The AP contends with the STAs based on the new backoff counterand accesses the channel.

In the embodiment of FIG. 11, the AP attempts to transmit the downlinkmulti-user data 352 after the failure of the UL-MU transmission process.That is, a downlink multi-user transmission interrupt (DL-MU interrupt)occurs, and the AP uses the next transmission opportunity for thedownlink multi-user transmission. The UL-MU transmission process hasfailed and the next contention window interval 44 starts after anextended IFS (EIFS) time. The AP and STAs contend in the contentionwindow interval 44, and STA5 wins the contention. Accordingly, the STA5transmits uplink data 340 to the AP, and the AP transmits an ACK 346 inresponse thereto. In the next contention window interval 46, the AP winsthe contention, and the AP transmits downlink multi-user data 352. TheSTAs receiving the downlink data 352 from the AP transmit a multiplexedblock ACK 358 in response thereto.

As described above, the AP performs a backoff procedure in the nextcontention intervals 44 and 46 based on a new backoff counter determinedin the increased contention window, and transmits downlink multi-userdata 352 when the backoff counter of the backoff procedure expires. Inthe embodiment of FIG. 11, the AP transmits the downlink multi-user data352 after the failure of the UL-MU transmission process. However, thepresent invention is not limited thereto, and the AP may transmitdownlink single-user data or a new trigger frame.

FIG. 12 illustrates an embodiment of a channel approach according toanother embodiment of the present invention. In the embodiment of FIG.12, duplicated descriptions of parts which are the same or correspondingto those of the embodiment of FIGS. 10 and 11 will be omitted.

According to the embodiment of FIG. 12, when the UL-MU transmissionprocess by the TF-R 312 has failed, the retransmission of the TF-R 312may not be performed and the size of the contention window may not beincreased. Due to the characteristic of random access, collisions mayoccur even in situations where traffic is not congested. Thus, uniformlyincreasing the size of a contention window may reduce the transmissionefficiency. Therefore, when the random access based UL-MU transmissionprocedure has failed, the AP may attempt the next transmission withoutincreasing the size of the contention window.

The AP may attempt any one of a downlink single-user transmission, adownlink multi-user transmission, and a transmission of a new triggerframe in the next contention window intervals 44 and 46. When attemptinga downlink single-user transmission or a downlink multi-usertransmission, the AP obtains a new backoff counter within the contentionwindow based on the access category of data to be transmitted. Whentransmitting a new trigger frame, the AP obtains a new backoff counterwithin the contention window based on the access category for thetrigger frame. The AP contends with the STAs based on the new backoffcounter and accesses the channel. The AP may obtain a new backoffcounter without increasing the size of the contention window even thoughthe previous UL-MU transmission process has failed.

In the embodiment of FIG. 12, the AP attempts to transmit downlinkmulti-user data 352 after the failure of the UL-MU transmission process.The AP and STAs contend in the contention window interval 44, and the APwins the contention. Thus, the AP transmits downlink multi-user data 352and the STAs transmit a multiplexed block ACK 358 in response thereto.In the next contention window interval 46, the STA5 wins the contention,and the STA5 transmits uplink data 340. The AP receiving the uplink data340 from the STA5 transmits an ACK 346 in response thereto.

FIGS. 13 to 15 illustrate an embodiment in which the embodiments ofFIGS. 10 to 12 are extended to a general uplink multi-user transmissionprocess. The trigger frame 314 indicates identification information ofeach STA participating in the UL-MU transmission and information ofallocated resource units. In the embodiments of FIGS. 10 to 12, thetrigger frame 314 solicits uplink multi-user data transmission of STA1,STA2 and STA3. However, the AP does not receive any uplink data inresponse to the trigger frame 314, and performs scheduling for thefailure of the UL-MU transmission process. In the embodiments of FIGS.13 to 15, duplicated descriptions of parts which are the same orcorresponding to those of the embodiments of FIGS. 10 to 12 will beomitted.

First, according to the embodiment of FIG. 13, the AP may performretransmission of the trigger frame 314 when the UL-MU transmissionprocess has failed. For the retransmission of the trigger frame 314, theAP obtains a new backoff counter. In this case, the new backoff countercan be determined within a range of twice the contention window used inobtaining the previous backoff counter. That is, when the UL-MUtransmission process has failed, the AP doubles the size of thecontention window to be used in the backoff procedure of the nextcontention window intervals 44 and 46. In the contention windowintervals 44 and 46, the AP performs a backoff procedure to retransmitthe trigger frame 314 based on the new backoff counter. Theretransmission of the trigger frame 314 may be performed until theretransmission is successful within the preset retransmission limit.

According to an embodiment of the present invention, the AP may selectan access category for transmitting the trigger frame 314. The size ofthe contention window for transmitting the trigger frame 314 isdetermined based on the selected access category. According to anembodiment of the present invention, a minimum contention window value,a maximum contention window value, an AIFS time, a maximum transmissionopportunity (TXOP), and the like may be defined for each accesscategory. Accordingly, the size of the contention window fortransmitting the trigger frame 314 is determined between the minimumcontention window value and the maximum contention window value set inthe corresponding access category. According to an embodiment, an accesscategory separately set for the trigger frame 314 may be used. Accordingto another embodiment of the present invention, any one of thecategories set for the enhanced distributed channel access (EDCA) may beselected as an access category for transmitting the trigger frame 314.

In the embodiment of FIG. 13, the AP determines the size of thecontention window based on the access category corresponding to thetrigger frame 314 and assigns a backoff counter within the determinedcontention window. When the UL-MU transmission process has failed andthe AP retransmits the trigger frame 314, the AP increases the size ofthe contention window of the access category for the trigger frame 314.According to an embodiment, the size of the increased contention windowis determined within a range of twice the size of the previouscontention window. A new backoff counter for retransmitting the triggerframe 314 is obtained within the increased contention window.

The AP and the STAs contend in the next contention window intervals 44and 46, and the terminal whose backoff counter has expired performstransmission. In this case, the AP participates in the contention usingthe new backoff counter. The AP that has won the contention in thecontention window interval 46 retransmits the trigger frame 314. Inresponse to the retransmitted trigger frame 314, STA1 and ST3 transmituplink multi-user data 323. Since uplink data has been received from atleast one of the STAs indicated by the trigger frame 314, the APdetermines that the UL-MU transmission procedure is successful.Accordingly, the AP transmits ACK information for STA1 and STA3 via theM-BA 330.

Next, according to the embodiment of FIG. 14, the AP may increase thesize of the contention window without performing the retransmission ofthe trigger frame 314 when the UL-MU transmission process has failed.When the UL-MU transmission process has failed, the AP may attempt anyone of a downlink single-user transmission, a downlink multi-usertransmission, and a transmission of a new trigger frame in the nextcontention window interval. In this case, the AP may increase thecontention window based on the access category of the packet to betransmitted. That is, when attempting a downlink single-usertransmission or a downlink multi-user transmission, the AP obtains a newbackoff counter by doubling the size of the contention window based onthe access category of data to be transmitted. When transmitting a newtrigger frame, the AP obtains a new backoff counter by doubling the sizeof the existing contention window based on the access category for thetrigger frame. The AP uses the new backoff counter determined based onthe increased contention window to contend with the STAs and access thechannel.

In the embodiment of FIG. 14, the AP attempts to transmit the downlinkmulti-user data 352 after the failure of the UL-MU transmission process.According to an embodiment of the present invention, a separate accesscategory for downlink multi-user transmission may be defined. In thiscase, the AP may obtain a new backoff counter by increasing the size ofthe contention window of the separate access category. According toanother embodiment of the present invention, the downlink multi-usertransmission may be performed based on a primary access category. Inthis case, the AP may obtain a new backoff counter by increasing thesize of the contention window of the primary access category.

Next, according to the embodiment of FIG. 15, the AP may not retransmitthe trigger frame 314 and may not increase the size of the contentionwindow when the UL-MU transmission process fails. That is, when theUL-MU transmission process has failed, the AP may attempt the nexttransmission without increasing the size of the contention window.

The AP may attempt any one of a downlink single-user transmission, adownlink multi-user transmission, or a transmission of a new triggerframe in the next contention window intervals 44 and 46. In this case,the AP may determine the contention window based on the access categoryof the packet to be transmitted. That is, when attempting a downlinksingle-user transmission or a downlink multi-user transmission, the APobtains a new backoff counter within a contention window based on theaccess category of data to be transmitted. When transmitting a newtrigger frame, the AP obtains a new backoff counter within the existingcontention window based on the access category for the trigger frame.The AP contends with the STAs based on the new backoff counter andaccesses the channel.

FIGS. 16 and 17 illustrate embodiments of channel access in an uplinkmulti-user transmission process using a wideband channel According to anembodiment of the present invention, the UL-MU transmission process maybe performed through a wideband channel of 20 MHz or more. In theembodiments of FIGS. 16 and 17, the duplicated descriptions of partswhich are the same or corresponding to those of the embodiments of FIGS.10 to 15 will be omitted.

In the embodiment of FIGS. 16 and 17, the AP transmits trigger frames314 a and 314 b on the primary channel and the secondary channel toinitiate the UL-MU transmission process. The AP may perform a backoffprocedure on the primary channel for transmitting the trigger frames 314a, 314 b. When the backoff procedure for transmitting the trigger frame314 a expires in the contention window interval 42, the AP transmits thetrigger frame 314 a on the primary channel. The AP performs a CCA forthe secondary channel during a PIFS time before the expiration of thebackoff procedure. When the secondary channel is in the idle state as aresult of the CCA, the AP transmits the trigger frame 314 a of theprimary channel and the trigger frame 314 b of the secondary channeltogether. In the embodiment of FIGS. 16 and 17, the trigger frame 314 aof the primary channel solicits uplink multi-user data transmission ofSTA1, STA2 and STA3, and the trigger frame 314 b of the secondarychannel solicits uplink multi-user transmission of STA4, STA5 and STA6.

Referring to FIG. 16, the AP does not receive any uplink data inresponse to the trigger frames 314 a and 314 b transmitted through theprimary channel and the secondary channel, and performs scheduling forthe failure of the UL-MU transmission process. According to theembodiment of FIG. 16, the AP may perform retransmission of the triggerframes 314 a and 314 b. For retransmitting the trigger frames 314 a and314 b, the AP obtains a new backoff counter. A specific embodiment forobtaining the new backoff counter for retransmitting the trigger frames314 a and 314 b is as described above in the embodiment of FIG. 13.

The AP and the STAs contend in the next contention window intervals 44and 46, and the terminal whose backoff counter has expired performstransmission. In this case, the AP participates in the contention usingthe aforementioned new backoff counter. The AP that has won thecontention in the contention window interval 46 retransmits the triggerframes 314 a and 314 b. In response to the retransmitted trigger frames314 a and 314 b, STA1 and ST3 transmit uplink multi-user data 322 a onthe primary channel, and STA4, STA5 and STA6 transmit uplink multi-userdata 322 b on the secondary channel. Since uplink data has been receivedfrom at least one of the STAs indicated by the trigger frames 314 a and314 b, the AP determines that the UL-MU transmission process issuccessful. Accordingly, the AP transmits the M-BA 330 a and 330 bincluding ACK information for the five STAs that have successfullytransmitted the uplink data.

Next, referring to FIG. 17, the AP receives uplink data 322 c of STA4 inresponse to the trigger frames 314 a and 314 b transmitted on theprimary channel and the secondary channel. Since uplink data 322 c hasbeen received from at least one of the STAs indicated by the triggerframes 314 a and 314 b, the AP determines that the UL-MU transmissionprocess is successful. The AP transmits the M-BA 330 a and 330 bincluding ACK information for the STA4 that has successfully transmittedthe uplink data.

Since the UL-MU transmission process has been determined to besuccessful, the AP does not increase the size of the contention windowto be used in the backoff procedure in the next contention windowinterval 44. That is, the AP obtains a new backoff counter within thecontention window determined based on the access category of the nextdata 352 a and 352 b to be transmitted. The AP contends with the STAsbased on the new backoff counter and accesses the channel.

<An EDCA Method of a Multi-User Transmission>

FIG. 18 illustrates an embodiment of an enhanced distributed channelaccess (EDCA). Referring to FIG. 18, data to be transmitted by aterminal is logically arranged in each access category queue accordingto a predetermined priority. The access category includes a voice accesscategory (i.e., AC_VO), a video access category (i.e., AC_VI), a besteffort access category (i.e., AC_BE) and a background access category(i.e., AC_BK). The terminal contends for channel access based on theparameters set for each access category. In this case, the parametersinclude a minimum contention window value, a maximum contention windowvalue, an AIFS time, and a maximum TXOP.

Each access category performs internal contention based on theparameters of the access category when the corresponding queue is notempty. That is, a backoff counter is assigned to a corresponding accesscategory based on the parameters of each access category, and internalcontention between the access categories is performed based on theassigned backoff counters. The access category whose backoff counterexpires first and has won the internal contention is set to the primaryaccess category and data in the queue of the corresponding accesscategory is determined as transmission data. According to an embodiment,in a multi-user transmission, data in a secondary access category may betransmitted with data in the primary access category using TXOP sharing.

In the embodiment of FIG. 18, data to be transmitted to STA2, STA4 andSATS are stacked in the queue of AC_VO, and data to be transmitted toSTA1 and STA3 are stacked in the queue of AC_VI. In addition, data to betransmitted to STA2 and STA3 are stacked in the queue of AC_BE.Therefore, AC_VO, AC_VI and AC_BE perform internal contention using therespective parameters. Hereinafter, in the embodiments of FIGS. 19 to24, it is assumed that AC_VI is set as a primary access category andAC_VO and AC_BE are set as a secondary access category as a result ofthe internal contention.

FIG. 19 illustrates an embodiment of a downlink multi-user transmissionprocess. In the embodiment of FIG. 19, the AP is a multi-usertransmitting terminal, and the STA, STA2, STA3 and STA4 are multi-userreceiving terminals.

The AP may perform a downlink multi-user data transmission when adownlink multi-user (DL-MU) interrupts occur. In the embodiment of thepresent invention, the DL-MU interrupt indicates an operation in which apredetermined condition for DL-MU data transmission is satisfied, and amulti-user transmitting terminal determines transmission of DL-MU data.The predetermined condition for the DL-MU interrupt to occur includes acase that data to be transmitted to a plurality of STAs is stacked apredetermined size or more in an access category queue, a case that apredetermined time or more has elapsed after data to be transmitted to aplurality of STAs is stacked in an access category queue, and the like.According to an embodiment of the present invention, the AP may generatea separate virtual queue for the DL-MU transmission. In this case, DL-MUinterrupts may occur when data of a predetermined size or more isstacked in the virtual queue.

When a DL-MU interrupt occurs, the AP performs a backoff procedure inthe contention window interval 52 for transmitting the downlinkmulti-user data 420. For the backoff procedure to transmit downlinkmulti-user data 420, the AP assigns a backoff counter. According to anembodiment, the AP may determine a contention window based on an accesscategory separately set for DL-MU transmission and assign a backoffcounter within the contention window. According to another embodiment,the AP may determine a contention window based on the primary accesscategory of the downlink multi-user data 420 to be transmitted and mayassign a backoff counter within the contention window. The AP performsthe backoff procedure in the contention window interval 52 using theassigned backoff counter after an AIFS time of the set access category.

When the backoff counter of the backoff procedure for transmitting thedownlink multi-user data 420 expires in the contention window interval52, the AP transmits downlink multi-user data 420. The downlinkmulti-user data 420 may be transmitted in a form including at least oneof OFDMA and MU-MIMO. The STAs receive downlink multi-user data 420transmitted by the AP and transmit ACK 430 in response thereto. In theembodiment of FIG. 19, the AP transmits downlink multi-user data 420 toSTA1, STA2, STA3 and STA4, and each STA transmits ACK 430 in response tothe reception of the downlink multi-user data 420. The ACK 430transmitted by a plurality of STAs may be transmitted by beingmultiplexed in a time domain or a frequency domain.

After the DL-MU transmission process is completed, the AP may performadditional data transmission in the next contention window intervals 54and 56. If a new DL-MU interrupt does not occur, the AP performs atransmission of downlink single-user data 440. In this case, downlinkdata 440 of an access category that has obtained the transmissionopportunity through internal contention between access category queuesof the AP may be transmitted. In the embodiment of FIG. 19, AC_VO haswon the internal contention of the AP. The AP transmits the downlinkdata 440 of AC_VO to the STA5 after a backoff procedure in thecontention window interval 54. The STA5 receives the downlink data 440and transmits an ACK 445 in response thereto.

Thereafter, when the DL-MU interrupt occurs again, the AP performs abackoff procedure for transmitting the downlink multi-user data 450 inthe contention window interval 56. When the backoff counter of thebackoff procedure expires, the AP transmits downlink multi-user data450. The AP transmits downlink multi-user data 450 to STA2 and STA3, andeach STA transmits ACK 455 in response to the reception of the downlinkmulti-user data 450.

FIGS. 20 to 22 illustrate a channel access method when a transmission ofsome data has failed in the downlink multi-user transmission process. Inthe embodiment of FIGS. 20 to 22, the AP transmits downlink multi-userdata 420 to STA1 through STA4. However, some downlink data, i.e.,downlink data of AC_BE to STA2, has failed to be transmitted. STA1 toSTA4 transmit an ACK 432 in response to the successfully receiveddownlink data. In each of the embodiments of FIGS. 20 to 22, duplicateddescriptions of parts which are the same or corresponding to those ofthe previous embodiments will be omitted.

First, referring to FIG. 20, the AP considers that the transmissionprocess is successful even when a part of data is successfullytransmitted in the transmission of downlink multi-user data 420. Thatis, when the ACK 432 is received from at least one STA among the STAs towhich the downlink multi-user data 420 is transmitted, the AP determinesthat the DL-MU transmission process is successful. On the other hand,the AP may attempt to retransmit some downlink data that has failed tobe transmitted. According to an embodiment, the AP may retransmitdownlink data that has failed to be transmitted through internalcontention. The downlink data that has failed to be transmitted contendfor transmission in the access category queue of the corresponding data.In the embodiment of FIG. 20, the downlink data 441 of the AC_BE thathas failed to be transmitted in the first DL-MU transmission process isretransmitted through the internal contention of the AP in the nextcontention window interval 54. STA2 receives downlink data 441 andtransmits ACK 446 in response thereto.

Next, according to the embodiment of FIG. 21, when a transmission ofsome data has failed in the transmission of the downlink multi-user data420, the AP increases the size of the contention window of the accesscategory of the corresponding data. According to an embodiment, the sizeof the contention window of the access category may be increased totwice the size of the previous contention window. By thus increasing thesize of the contention window of the access category, a penalty may beadded to the contention when retransmitting the downlink data that hasfailed to be transmitted. This penalty for the transmission contentionmay be only applied to the internal contention of the AP. That is, theAP does not increase the sizes of the contention windows of the accesscategories other than the access category of the downlink data to beretransmitted.

Referring to the embodiment of FIG. 21, in the first DL-MU transmissionprocess, transmission of downlink data of AC_BE has failed. Thus, the APincreases the size of the contention window of AC_BE. AC_BE obtains anew backoff counter within the increased contention window andparticipates in the internal contention using the new backoff counter.In the next contention window interval 54, AC_VO has won the internalcontention of the AP. The AP transmits the downlink data 440 of AC_VO tothe STA5 after a backoff procedure in the contention window interval 54.The STA5 receives the downlink data 440 and transmits an ACK 445 inresponse thereto.

After the transmission of downlink data 440, a DL-MU interrupt occursand the AP performs a backoff procedure for transmitting downlinkmulti-user data 452 in the contention window interval 56. When thebackoff counter of the backoff procedure expires, the AP transmits thedownlink multi-user data 452. The data of AC_BE that has received a newbackoff counter and has contended for transmission is transmitted asdownlink multi-user data 452. The STAs receiving the downlink multi-userdata 452 from the AP transmit an ACK 457 in response thereto.

Next, according to the embodiment of FIG. 22, when a transmission ofsome data has failed in the transmission of downlink multi-user data420, the AP decreases the size of the contention window of the accesscategory of the corresponding data. According to an embodiment, the sizeof the contention window of the access category may be set to theminimum contention window value of the corresponding access category.According to another embodiment, the size of the contention window ofthe access category may be reduced to a certain percentage value of thesize of the previous contention window. By thus decreasing the size ofthe contention window of the access category, a priority may be added tothe contention when retransmitting the downlink data that has failed tobe transmitted.

Referring to the embodiment of FIG. 22, the AP decreases the size of thecontention window of AC_BE in which the transmission has failed in thefirst DL-MU transmission process. AC_BE obtains a new backoff counterwithin the reduced contention window and participates in the internalcontention using the new backoff counter. As a result, AC_BE has won theinternal contention of the AP in the next contention window interval 54.The AP retransmits the downlink data 441 of AC_BE to the STA2 after abackoff procedure in the contention window interval 54. The STA2receives the downlink data 441 and transmits an ACK 446 in responsethereto.

On the other hand, according to an additional embodiment of the presentinvention, the size of the contention window of the access category thathas failed in transmission can be variously adjusted. For example, theincrease or decrease ratio of the size of the contention window may beadjusted according to the number of users or the size of channel thathas failed in transmission in the previous DL-MU transmission process.

FIGS. 23 and 24 illustrate a channel access method when a datatransmission of a primary access category has failed in the downlinkmulti-user transmission process. In the embodiment of FIGS. 23 and 24,the AP transmits downlink multi-user data 420 to STA1 through STA4.However, some downlink data, i.e., downlink data of AC_VI to STA1, hasfailed to be transmitted. In this case, the AC_VI is the primary accesscategory of the downlink multi-user data 420. STA2 to STA4 transmit anACK 434 in response to the successfully received downlink data. In eachof the embodiments of FIGS. 23 and 24, duplicated descriptions of partswhich are the same or corresponding to those of the previous embodimentswill be omitted.

According to an embodiment of the present invention, the EDCA of thedownlink multi-user data 420 may be performed based on the primaryaccess category of the corresponding data. That is, the AP determines acontention window based on the primary access category of the downlinkmulti-user data 420 and assigns a backoff counter within the contentionwindow. The AP performs a backoff procedure in the contention windowinterval 52 using the assigned backoff counter after an AIFS time of theset access category.

In addition, success of the transmission of the downlink multi-user data420 may be determined based on whether or not the primary accesscategory data is successfully transmitted. That is, when thetransmission of the primary access category data among the downlinkmulti-user data 420 is successful, the AP determines that the DL-MUtransmission process is successful. However, when the transmission ofthe primary access category data among the downlink multi-user data 420has failed, the AP determines that the DL-MU transmission process hasfailed. The AP performs retransmission of the primary access categorydata that has failed to be transmitted. When a DL-MU interrupt occursduring the retransmission, the AP may transmit the primary accesscategory data along with other data remaining in the queue via DL-MU.

Referring to FIG. 23, the AP determines a contention window of a primaryaccess category AC_VI whose transmission has failed based on theparameters of the corresponding access category and attempts aretransmission by assigning a new backoff counter. In the nextcontention window interval 54, the primary access category AC_VI winsthe contention, and the AP retransmits the downlink data 441 of theprimary access category AC_VI to the STA1. The STA1 receives thedownlink data 441 and transmits an ACK 446 in response thereto.

On the other hand, referring to FIG. 24, the AP may increase the size ofthe contention window due to the failure of the DL-MU transmissionprocess. According to an embodiment, the AP may increase the size of thecontention window of the primary access category AC_VI whosetransmission has failed. According to another embodiment, the AP mayincrease the sizes of the contention windows of the entire accesscategories. The AP obtains a new backoff counter within the increasedcontention window and performs retransmission of the downlink data 441of the primary access category AC_VI using the new backoff counter.

Meanwhile, according to another exemplary embodiment of the presentinvention, a contention window may be determined based on an accesscategory separately set for a DL-MU transmission, and a transmission ofdownlink multi-user data 420 may be performed by assigning a backoffcounter within the corresponding contention window. However, success ofthe transmission of the downlink multi-user data 420 may be determinedbased on whether or not the primary access category data has beensuccessfully transmitted. In this case, the AP may performretransmission of the downlink data 441 of the primary access categorybased on the parameters of the access category separately set for theDL-MU transmission.

FIGS. 25 and 26 illustrate an embodiment of an EDCA including amulti-user transmission. According to an embodiment of the presentinvention, the access category queue for EDCA may further comprise anaccess category queue for multi-user transmission. In this case, theaccess category queue for the multi-user transmission includes at leastone of a queue for a multi-user downlink transmission and a queue fortransmitting a trigger frame. According to an embodiment, the queue forthe multi-user transmission may be operated as a virtual queue.

Referring to FIG. 25, the AP inserts a virtual frame into an accesscategory AC_MU queue for the multi-user transmission. The AP determinesthe size of the contention window of the access category based on theparameters of AC_MU. According to an embodiment, the AC_MU may have ahigher priority than the access categories of data to be transmitted toanother STA. For example, the AC_MU may be set to have a higher prioritythan other access categories where the queue is not empty. Theparameters of AC_MU are determined based on the set priority. Accordingto another embodiment of the present invention, the parameters of AC_MUmay use parameters of a particular access category selected from otheraccess categories. For example, the parameters of AC_MU may be set equalto parameters of the highest priority access category among the otheraccess categories where the queue is not empty. The AC_MU may use one ofthe access categories used in the legacy WLAN system equally. Accordingto an embodiment, the parameters of the AC_MU may be set equal to theparameters of AC_VO or AC_VI to assign a priority to the UL-MUtransmission. As described above, the parameters of the access categoryinclude at least one of a minimum contention window value, a maximumcontention window value, an AIFS time, and a maximum TXOP.

According to still another embodiment of the present invention, the APmay adjust the parameters of AC_MU to adjust the priority of a frame ofthe corresponding queue in the internal contention. When the multi-usertransmission is performed with the highest priority such as the DL_MUinterrupt, the AP may set the contention window value of the AC_MU tozero. However, when multi-user transmission is not performed, the AP mayset the minimum contention window value or the contention window valueof the AC_MU to the maximum value of the system. The AP may adjust theparameters of AC_MU based on the system status or the state of the AC_MUqueue.

Referring to FIG. 26, the AP receives the buffer status report (BSR) ofSTAs and generates an AC_MU queue using the received buffer statusreport. According to an embodiment of the present invention, the AC_MUincludes an access category for transmitting a trigger frame. The AP mayperform a backoff procedure for transmitting the trigger frame when thereceived buffer status report information is a predetermined amount ormore. In this case, the AP determines the size of the contention windowbased on the parameters of the AC_MU and obtains a backoff counter fortransmitting the trigger frame within the determined contention window.

According to the embodiment of FIG. 26, the AP determines whether totransmit a trigger frame based on an internal contention between accesscategory queues for a downlink single-user transmission and an AC_MUqueue. More specifically, the access category queues for the downlinksingle-user transmission includes an AC_VO queue, an AC_VI queue, anAC_BE queue, and an AC_BK queue used in a legacy WLAN system. The APassigns backoff counters corresponding to the access category queues andthe AC_MU queue, respectively. In this case, the backoff counters areassigned based on the parameters of the access category set in thecorresponding queue, respectively. The AP may transmit a trigger framewhen the backoff counter corresponding to the AC_MU queue expires. Asdescribed above, according to the embodiment of the present invention,the AC_MU may have a higher priority than the access categories of datato be transmitted to another STA. Thus, the trigger frame may betransmitted with a higher priority than the frame of the other accesscategories.

Although the present invention is described by using the wireless LANcommunication as an example, the present invention is not limitedthereto and the present invention may be similarly applied even to othercommunication systems such as cellular communication, and the like.Further, the method, the apparatus, and the system of the presentinvention are described in association with the specific embodiments,but some or all of the components and operations of the presentinvention may be implemented by using a computer system having universalhardware architecture.

The detailed described embodiments of the present invention may beimplemented by various means. For example, the embodiments of thepresent invention may be implemented by a hardware, a firmware, asoftware, or a combination thereof.

In case of the hardware implementation, the method according to theembodiments of the present invention may be implemented by one or moreof Application Specific Integrated Circuits (ASICSs), Digital SignalProcessors (DSPs), Digital Signal Processing Devices (DSPDs),Programmable Logic Devices (PLDs), Field Programmable Gate Arrays(FPGAs), processors, controllers, micro-controllers, micro-processors,and the like.

In case of the firmware implementation or the software implementation,the method according to the embodiments of the present invention may beimplemented by a module, a procedure, a function, or the like whichperforms the operations described above. Software codes may be stored ina memory and operated by a processor. The processor may be equipped withthe memory internally or externally and the memory may exchange datawith the processor by various publicly known means.

The description of the present invention is used for exemplification andthose skilled in the art will be able to understand that the presentinvention can be easily modified to other detailed forms withoutchanging the technical idea or an essential feature thereof. Thus, it isto be appreciated that the embodiments described above are intended tobe illustrative in every sense, and not restrictive. For example, eachcomponent described as a single type may be implemented to bedistributed and similarly, components described to be distributed mayalso be implemented in an associated form.

The scope of the present invention is represented by the claims to bedescribed below rather than the detailed description, and it is to beinterpreted that the meaning and scope of the claims and all the changesor modified forms derived from the equivalents thereof come within thescope of the present invention.

INDUSTRIAL APPLICABILITY

Various exemplary embodiments of the present invention have beendescribed with reference to an IEEE 802.11 system, but the presentinvention is not limited thereto and the present invention can beapplied to various types of mobile communication apparatus, mobilecommunication system, and the like.

The invention claimed is:
 1. A base wireless communication terminalcomprising: a transceiver configured to transmit and receive wirelesssignals; and a processor configured to process wireless signalstransmitted or received through the transceiver, wherein the processoris configured to: perform a backoff procedure for transmission of atrigger frame, wherein the backoff procedure is performed within acontention window for the transmission of the trigger frame, transmitthe trigger frame, to at least one of terminal, based on the backoffprocedure, and retransmit the trigger frame according to whether thetransmission of the trigger frame is failed, wherein whether thetransmission of the trigger frame is failed is determined based onwhether at least one physical layer protocol data unit (PPDU) isreceived in response to the trigger frame from the at least one ofterminal, and wherein a size of the contention window is adjusted toretransmit the trigger frame, when the transmission of the trigger frameis failed.
 2. The base wireless communication terminal of claim 1,wherein the transmission of the trigger frame is determined to fail,when the at least one PPDU is not received in response to the triggerframe from the at least one terminal.
 3. The base wireless communicationterminal of claim 2, wherein the size of the contention window isincreased, when the transmission of the trigger frame is failed, andwherein the trigger frame is retransmitted according to a back-offprocedure based on the increased contention window.
 4. The base wirelesscommunication terminal of claim 1, wherein the transmission of thetrigger frame is determined to success, when the at least one PPDU isreceived in response to the trigger frame from the at least oneterminal.
 5. The base wireless communication terminal of claim 4,wherein the size of the contention window is not increased, when thetransmission of the trigger frame is successful.
 6. The base wirelesscommunication terminal of claim 1, wherein the size of the contentionwindow is determined between a minimum contention window size and amaximum contention window size of an access category determined by theterminal for the back-off procedure.
 7. The base wireless communicationterminal of claim 1, wherein the trigger frame is transmitted withdownlink data.
 8. The base wireless communication terminal of claim 7,wherein the access category is a primary access category for thedownlink data.
 9. A wireless communication method of a base wirelesscommunication terminal, the method comprising: performing a backoffprocedure for transmission of a trigger frame; wherein the backoffprocedure is performed within a contention window for the transmissionof the trigger frame, transmit the trigger frame, to at least one ofterminal, based on the backoff procedure; and retransmit the triggerframe according to whether the transmission of the trigger frame isfailed, wherein whether the transmission of the trigger frame is failedis determined based on whether at least one physical layer protocol dataunit (PPDU) is received in response to the trigger frame from the atleast one of terminal, and wherein a size of the contention window isadjusted to retransmit the trigger frame, when the transmission of thetrigger frame is failed.
 10. The base wireless communication method ofclaim 9, wherein the transmission of the trigger frame is determined tofail, when the at least one PPDU is not received in response to thetrigger frame from the at least one terminal.
 11. The base wirelesscommunication method of claim 10, wherein the size of the contentionwindow is increased, when the transmission of the trigger frame is fail,and wherein the trigger frame is retransmitted according to a back-offprocedure based on the increased contention window.
 12. The basewireless communication method of claim 9, wherein the transmission ofthe trigger frame is determined to success, when the at least one PPDUis received in response to the trigger frame from the at least oneterminal.
 13. The base wireless communication method of claim 12,wherein the size of the contention window is not increased, when thetransmission of the trigger frame is successful.
 14. The base wirelesscommunication method of claim 9, wherein the size of the contentionwindow is determined between a minimum contention window size and amaximum contention window size of an access category determined by theterminal for the back-off procedure.
 15. The base wireless communicationmethod of claim 9, wherein the trigger frame is transmitted withdownlink data.
 16. The base wireless communication method of claim 15,wherein the access category is a primary access category for thedownlink data.